INTRODUCTION
Newer anaesthetic drugs introduced in practice and older drugs for novel route or
indication has led to the need to determine the optimal dose. In this context, two
most recent studies, for example, estimated the effective dose of opioids for epidural
initiation in the latent and active phases during the first stage of labour and remimazolam
bolus for anaesthesia induction in different age groups.[1
2] The primary aim of these studies was to estimate a quantile, a dose at which a
desired probability of response is achieved. This dose is called an effective dose
(ED) with quantile g; that is, EDg is defined as the dose of a drug that produces
a response of interest at quantile ‘g’ (‘g’ may be 90, 95 or 99%) of the study population.
In these studies, doses cannot be assigned randomly because some patients may receive
optimal low doses, whereas others may receive high doses that might induce adverse
effects. To address this issue, scholars demonstrated the up-and-down design (UDM)
to estimate the median, whereas Derman demonstrated through nonparametric experimentation
that dose levels could be centred around any given target quantile using the up-and-down
designs using a biased coin.[3
4] Later, Durham et al.[5] generalised UDM as a biased coin design to estimate EDg
using random walk rules. This method of the biased coin up-and-down design (BCUD)
is used to assign doses sequentially by random walk rule in which efficacy or toxicity
is assumed to be monotonically related to dose.
In dose-finding studies within the BCUD setting, the isotonic regression technique
is utilised to estimate the effective dose for a particular drug (EDg). Unlike simple
linear regression, isotonic regression ensures that the regression function is monotonic,
meaning it continuously increases or decreases. This method is suitable because it
assumes that increasing the dose level increases the drug effect.[6] In this regression,
the dose (x) serves as the predictor variable, whereas the response probability (P
(x) = P (response|dose x)) acts as the dependent variable, representing the probability
of a response to the drug.
This paper aims to describe these mathematical concepts using a diagrammatic explanation,
specifically focusing on a section of the isotonic regression curve [Figure 1]. The
paper also covers the estimation of naïve probability and adjusted probability using
the pooled adjacent violators algorithm (PAVA) for the response.
Figure 1
A small section of isotonic regression curve
The data used to elucidate the calculations needed to estimate ED90 and its 95% confidence
interval (CI) are given [Table 1 and Figure 2a] in annexure.
Table 1
Norepinephrine prophylactic bolus dose and the response of 40 successive women
Patient number
Norepinephrine Prophylactic bolus dose (μg)
Response#
Patient number
Norepinephrine Prophylactic bolus dose (μg)
Response#
1
4
F
21
11
S
2
5
F
22
11
F
3
6
F
23
12
S
4
7
S
24
12
S
5
7
S
25
12
S
6
7
S
26
12
S
7
7
S
27
12
S
8
7
S
28
12
S
9
7
F
29
12
S
10
8
S
30
12
S
11
8
S
31
11
S
12
8
F
32
11
S
13
9
S
33
11
S
14
9
S
34
11
S
15
9
S
35
11
S
16
9
F
36
11
S
17
10
F
37
11
S
18
11
S
38
11
S
19
11
S
39
11
S
20
11
S
40
11
S
#
F is failure, and S is success
Figure 2
Plots showing the (a) patient’s allotment sequence and the response to the assigned
dose of norepinephrine prophylactic bolus (μg) and (b) naïve probability (observed
response rate) and PAVA probability (adjusted response rate) for the assigned dose
Estimation of naive and PAVA probability
Table 2 shows the dose assigned to the participants (nDoses), number of patients (nTrials),
number of successes (nEvents), the naïve probability and PAVA adjusted response rate
at each distinctive dose level.
Table 2
Naive (observed) and PAVA probability and bootstrap estimates of 3000 boot replications
nDoses
nTrials
nSuccess
Naive probability
PAVA probability
4
1
0
0
0
5
1
0
0
0
6
1
0
0
0
7
6
5
0.8333
0.7143
8
3
2
= 0.6667
0.7143
9
4
3
= 0.75
0.7143
=10
1
0
0
f ()=0.7143
=11
15
14
0.9333
f ()=0.9333
12
8
8
1
1
Statistic from bootstrapping (3000 replications)
Value
Original statistic (ED90)
10.848
Mean of ED90’s of Boot Replications
10.772
Median of ED90’s of Boot Replications
10.834
Bias of Original statistic (ED90) (= Original Statistics – Mean of ED90’s of Boot
Replications)
-0.076
Bias Correction: (No. of Boot Replicates <=Original Statistic)/(Total Replicates +1)
0.51583
Standard error of Boot Statistic (Mean of ED90’s)
0.626
0.03969
zα/2
-1.96
z(1-α/2)
1.96
2.5% Bias-corrected Lower Bound
9.25
97.5% Bias-corrected Upper Bound
11.675
The naive probability is calculated [Table 2] using the following formula:
The PAVA probability is estimated using the PAVA algorithm for each dose level.[6]
Under the PAVA algorithm, starting with the lowermost dose, we have to find the first
adjoining pair of naive probability that violates the increasing ordering restriction
(i.e. increasing dose level increases the drug effect), that is where . The PAVA
probability for that pair of doses is
In Figure 2b, the first adjoining pair of doses that violate the ordering restriction
of naive probability is 7 and 8 μg, and the next successive pair of such doses is
9 and 10 μg, but for dose 10 μg, the naive probability is zero. Hence, the PAVA probability
using equation (B) for doses 8 and 9 μg is
The same PAVA probability of 0.7143 is taken for preceding and succeeding doses of
7 and 10 μg [Table 2].
ED90 and its 95% confidence interval
By substituting the values from Table 2 in annexure equation (8), the ED90 is given
below:
The precision of ED90 is its 95% CI and is estimated using annexure equations (11)
and (12):
The final estimation of 95% bias-corrected bootstrap confidence interval (BCBCI) is
estimated using the annexure equation (13):
Hence, the 95% CI for ED90 is (9.25, 11.675) μg.
All calculations were performed using R 4.2.1 (R Foundation of Statistical Computing,
Vienna, Austria), and the code used is given in Appendix I.
The bias of the original statistic (ED90) = -0.076 is basically due to the discrete
nature of the doses rather than the dimensional. The bias would be closer to zero
if the dose ranges continuously, say, 10.1, 10.2, 10.3,…, 11.0, which would make the
distance closer between the PAVA probability 0.7143 and 0.9000 for the dose of 10
μg.
DISCUSSION
We explored the application of isotonic regression for estimating EDg along with its
95% confidence interval in the context of dose-finding studies. In anaesthesia, it
is vital to assess how drug effects change with increasing doses using dose–response
characterisation. In a general scenario, we may encounter a violation of the assumption
of monotonicity in the observed probability, as occurred in the specified example.
We employed the PAVA algorithm to rectify this violation and ensure the validity of
the assumption.
The isotonic estimate of ED90 in the BCUD setting has low bias and variance, particularly
at low or high quantiles.[7] It also has a smaller mean square error than estimators
from other methods.[8
9
10] We also assessed the overfitting of the estimate using bootstrapping, and the
bias of the ED90 estimate was very low [-0.076, Table 2].
To evaluate the precision of the estimated target dose, several methods estimate the
bootstrap confidence interval with a confidence level of (1-α)*100%.[11] Simulation
studies suggested that the BCBCI method, described in the paper, provides better balance,
increased type I error and higher power than other bootstrap methods.[11
12]
Sequential dose-finding studies are appealing because they provide accurate and stable
estimates with small sample sizes ranging from 20 to 40 patients.[5
8] These studies determine the critical intensity level (dose) at which a drug either
produces or prevents a reaction in each patient. In such studies, the dose increment
between the first dose at experimentation and the following subsequent doses is very
small, and therefore, the outliers are unlikely.
The comparative analysis of the proposed technique against other methods is not included
in this paper as it is beyond the scope of the paper and can be found elsewhere.[8]
However, one notable comparison is the simplicity of the BCUD method compared to other
methods; for example, the continual reassessment method requires a mathematical model
to assign the dose and analyse previous dose responses for the next dose, which requires
the involvement of a biostatistician.[13] On the other hand, the BCUD sequential method
requires less computation and does not rely on a mathematical model to assign the
doses. It also has simple statistical properties for estimating the target EDg.[6]
In contrast, other standard methods like logit and probit regression are complex and
hence not widely used by anaesthesiologists. The isotonic estimate of EDg, on the
other hand, does not demand technical expertise, and that is why anaesthesiologists
should prefer this method to estimate any target dose without the help of the biostatistician.
CONCLUSION
The isotonic regression adjusts the observed response rate (naive) using PAVA when
it is not monotonically increasing with increasing dose levels. Also, it is straightforward
to estimate the effective dose for a target quantile in the BCUD setting. Additionally,
computer programming is needed only to estimate the 95% CI of EDg.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.